On the screenshot it’s highlighted, but that does not mean it was a match for that keyword. The highlighter script runs during the page load, after the query is completed. It is much more likely that either the other word, or a partial was matched somewhere and the highlighter highlights it as it only sees the search phrase – not the matching (it’s impossible).
Cyrillic should work the same way, it shouldn’t affect it.
If you want I can investigate this further, just make sure to add temporary back-end and FTP details, and I will run a debugging session.